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The present invention relates to therapeutic radiation sources, and more particularly to a reduced power, increased efficiency miniaturized radiation source that utilizes an optically driven thermionic cathode.
In the field of medicine, therapeutic radiation such as x-ray radiation and xcex3-ray radiation is used for diagnostic, therapeutic and palliative treatment of patients. The conventional medical radiation sources used for these treatments include large, fixed position machines as well as small, transportable radiation generating probes. The current state-of-the-art treatment systems utilize computers to generate complex treatment plans.
Conventional radiation systems used for medical treatment utilize a high power remote radiation source, and direct a beam of radiation at a target area, such as a tumor inside the body of a patient. This type of treatment is referred to as teletherapy because the radiation source is located a predefined distance from the target. This treatment suffers from the disadvantage that tissue disposed between the radiation source and the target is exposed to radiation. Teletherapy radiation sources, which apply radiation to target regions internal to a patient from a source external to the target regions, often cause significant damage not only to the target region or tissue, but also to all surrounding tissue between the entry site, the target region, and the exit site.
Brachytherapy, on the other hand, is a form of treatment in which the source of radiation is located close to or in some cases within the area receiving treatment. Brachytherapy, a word derived from the ancient Greek word for close (xe2x80x9cbrachyxe2x80x9d), offers a significant advantage over teletherapy, because the radiation is applied primarily to treat only a predefined tissue volume, without significantly affecting the tissue adjacent to the treated volume. The term brachytherapy is commonly used to describe the use of a radioactive xe2x80x9cseed,xe2x80x9d i.e. encapsulated radioactive isotopes which can be placed directly within or adjacent the target tissue to be treated. Handling and disposal of such radioisotopes, however, may impose considerable hazards to both the handling personnel and the environment.
The term xe2x80x9cx-ray brachytherapyxe2x80x9d is defined for purposes of this application as x-ray radiation treatment in which the x-ray source is located close to or within the area receiving treatment. An x-ray brachytherapy system, which utilizes a miniaturized low power radiation source that can be inserted into, and activated from within, a patient""s body, is disclosed in U.S. Pat. No. 5,153,900 issued to Nomikos et al., U.S. Pat. No. 5,369,679 to Sliski et al., and U.S. Pat. No. 5,422,926 to Smith et al., all owned by the assignee of the present application, all of which are hereby incorporated by reference. The x-ray brachytherapy system disclosed in the above-referenced patents includes a miniaturized, insertable probe which is capable of generating x-ray radiation local to the target tissue, so that radiation need not pass through the patient""s skin, bone, or other tissue prior to reaching the target tissue. The insertable probe emits low power x-rays from a nominal xe2x80x9cpointxe2x80x9d source located within or adjacent to the desired region to be affected. In x-ray brachytherapy, therefore, x-rays can be applied to treat a predefined tissue volume without significantly affecting the tissue adjacent to the treated volume. Also, x-rays may be produced in predefined dose geometries disposed about a predetermined location. X-ray brachytherapy offers the advantages of brachytherapy, while avoiding the use and handling of radioisotopes. Also, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.
X-ray brachytherapy typically involves positioning the insertable probe into or adjacent to the tumor, or into the site where the tumor or a portion of the tumor was removed, to treat the tissue adjacent the site with a local boost of radiation. X-ray probes of the type generally disclosed in U.S. Pat. No. 5,153,900 include a housing, and a hollow, tubular probe or catheter extending from the housing along an axis and having an x-ray emitting target at its distal end. The probe may enclose an electron source, such as a thermionic cathode. In another form of an x-ray brachytherapy device, as disclosed in U.S. Pat. No. 5,428,658, an x-ray probe may include a flexible probe, such as a flexible fiber optic cable enclosed within a metallic sheath. The x-ray probe may also include a substantially rigid capsule that is coupled to a distal end of the flexible probe. The capsule encloses an electron source and an x-ray emissive target element. The electron source may be a photocathode. In a photocathode configuration, a photoemissive substance is irradiated by a LED or a laser source, causing the generation of free electrons. Typically, the flexible fiber optic cable couples light from a laser source or a LED to the photocathode.
In the devices disclosed in U.S. Pat. Nos. 5,133,900 and 5,428,658, an accelerating electric field may be established between the electron source and the target element. The established electric field acts to accelerate the electrons emitted from the electron source toward the target element. The target element emits radiation in response to incident electrons from the electron source.
In one form of a conventional thermionic cathode, a filament is heated resistively with a current. This in turn heats the cathode so that electrons are generated by thermionic emission. In one form of a conventional x-ray machine that uses such resistively heated thermionic cathodes, the cathode assembly may consist of a thoriated tungsten coil approximately 2 mm in diameter and 1 to 2 cm in length. When resistively heated with a current of 4 A or higher, the thoriated tungsten coil thermionically emits electrons. In one configuration, this coil is surrounded by a metal focusing cup which concentrates the beam of electrons to a small spot on an opposing anode which also functions as the target. The beam is focused on the anode to a spot diameter, usually ranging anywhere from about 0.3 to 2.5 millimeters. In many applications, most of the energy from the electron beam is converted into heat at the anode. To accommodate such heating, high power medical x-ray sources often utilize liquid cooling and a rapidly rotating anode. An increased effective target area is thereby established, permitting a small focal spot while minimizing the effects of localized heating.
To achieve good thermal conductivity and effective heat dissipation, the anode typically is fabricated from copper. In addition, the area of the anode onto which an electron beam is incident must be made from a material of high atomic number, in order for x-rays to be generated efficiently. To meet the requirements of thermal conductivity, effective heat dissipation, and efficient x-ray generation, a tungsten alloy is typically embedded in the copper.
It is desirable that the electron source be heated as efficiently as possible, namely that the thermionic cathode reach as high a temperature as possible using as little power as possible. In conventional x-ray tubes, for example, thermal vaporization of the tube""s coiled cathode filament is frequently responsible for tube failure. Also, the anode heated to a high temperature can cause degradation of the radiation output. During relatively long exposures from an x-ray source, e.g. during exposures lasting from about 1 to about 3 seconds, the anode temperature may rise sufficiently to cause it to glow brightly, accompanied by localized surface melting and pitting which degrades the radiation output.
While a photocathode avoids such problems, one disadvantage of using a photocathode is the difficulty in fabricating the photocathode. A photocathode must have a sufficient quantum efficiency, where quantum efficiency relates to the number of electrons generated per incident light quantum. The degree of efficiency must be balanced to the intensity of available incident light. For practical substances, with reasonable quantum efficiencies above 10xe2x88x923, the fabrication of the photocathode should be performed in a vacuum. As disclosed in U.S. Pat. No. 5,428,658, owned by the assignee of the present application and hereby incorporated by reference, in one form the vacuum fabrication can be carried out with the fiber optic cable positioned in a bell jar. By way of example, an Agxe2x80x94Oxe2x80x94Cs photosurface can be fabricated in the conventional manner. Subsequently, without exposure to air, the fiber optic cable can be inserted into the tubular probe, and the end of the fiber optic cable can be vacuum sealed to the probe.
It is an object of this invention to provide an increased efficiency, miniaturized radiation source having significantly reduced power requirements. It is another object of this invention to provide a miniaturized radiation source in which the electron source can generate electrons with minimal heat loss, without requiring a vacuum-fabricated photocathode. It is yet another object of this invention to provide a miniaturized radiation source in which laser energy is used to heat a thermionic cathode, instead of heating a thermionic cathode via conventional ohmic heating. In this way, electrons can be produced in a quantity sufficient to form an electron current necessary for generating therapeutic radiation at the target, while significantly reducing the requisite power requirements for the radiation source.
In order to reduce the power requirements for the laser-heated therapeutic radiation source discussed above, it is necessary to minimize heat loss by the thermionic cathode. Heat loss in a laser-heated thermionic cathode includes 1) heat lost by thermal conduction; 2) heat loss caused by the portion of the incident laser radiation that remains unabsorbed; and 3) heat loss by thermal radiation. It is yet another object of this invention to increase the efficiency of a laser-heated thermionic cathode in a radiation source, by reducing the amount of heat that is lost due to incident laser radiation that remains unabsorbed by the thermionic cathode.
The present invention is directed to a miniaturized source of therapeutic radiation having a low power, electron-beam activated radiation source. In particular, the apparatus of the present invention includes a thermionic cathode heated by a source of optical radiation, preferably a laser. By using a laser to heat a thermionic cathode to an electron emitting temperature, the power requirements for the therapeutic radiation source are significantly reduced. Therapeutic radiation generated by the apparatus of the present invention may include, but is not limited to, x-rays. In medical applications, the apparatus may be fully or partially implanted into, or surface mounted onto a desired area of a host, so as to irradiate a pre-selected region with therapeutic radiation. The apparatus of the present invention can operate at a relatively low voltage, for example in the range of approximately 10 keV to 90 keV, with electron currents for example in the range of from approximately 1 nA to about 1 mA.
A therapeutic radiation source in accord with the present invention includes a radiation generator assembly, a source of optical radiation, and a probe assembly. The source of optical radiation is preferably a laser that generates a substantially monochromatic, coherent beam of radiation. The radiation generator assembly includes an electron source for emitting electrons to generate an electron beam along a beam path, and a target element positioned in the beam path. The electron source is preferably a thermionic cathode having an electron emissive surface. The target element includes means for emitting therapeutic radiation in response to incident accelerated electrons from said electron beam. In a preferred embodiment, the target element is spaced apart from and opposite the electron emissive surface of the thermionic cathode. The target element includes at least one radiation emissive element adapted to emit therapeutic radiation in response to incident accelerated electrons from the thermionic cathode. The therapeutic radiation source also includes means for providing an accelerating voltage so as to establish an accelerating electric field which acts to accelerate electrons emitted from said electron source toward the target element.
In one embodiment, the radiation generator assembly further includes a substantially rigid capsule which encloses the electron source and the target element. Preferably, the electron source is at its proximal end, and the target element is at its distal end. The capsule is evacuated and preferably includes a radiation transmissive region.
The probe assembly includes an optical delivery structure, preferably a fiber optic cable, having a proximal end and a distal end. The distal end of the fiber optic cable is coupled to the radiation generator assembly. The fiber optic cable transmits optical radiation, generated by the source and incident on the proximal end, to the distal end. The fiber optic cable directs a beam of the transmitted optical radiation to impinge upon a surface of the thermionic cathode, wherein the beam of optical radiation has a power level sufficient to heat at least a portion of a surface of the thermionic cathode to an electron emitting temperature so as to cause thermionic emission of electrons from the surface. In one embodiment, the probe assembly includes a flexible metallic sheath enclosing the fiber optic cable.
In one embodiment, the means for providing the accelerating voltage is a power supply having a first terminal and a second terminal, and having drive means for establishing an output voltage between the first terminal and the second terminal. In one form, the power supply may be electrically coupled to the target element by way of the first and second terminals. The first terminal of the power supply can be electrically coupled to the electron emissive surface of the thermionic cathode, and the second terminal electrically coupled to the target element, thereby establishing an electric field which accelerates electrons emitted from the thermionic cathode toward the target element.
In a preferred embodiment, the apparatus of the present invention includes one or more reflector elements disposed at predetermined locations along an inner surface of the housing. The reflector elements are operative to reflect incident laser radiation unabsorbed by the thermionic cathode back to the thermionic cathode, thereby increasing the efficiency of the therapeutic radiation source.